Seawater carbon chemistry and feeding and growth of juvenile crownof-thorns starfish, Acanthaster planci

The indirect effects of changing climate in modulating trophic interactions can be as important as the direct effects of climate stressors on consumers. The success of the herbivorous juvenile stage of the crown-of-thorns starfish (COTS), Acanthaster planci, may be affected by the impacts of ocean conditions on its crustose coralline algal (CCA) food. To partition the direct effects of near future ocean acidification on juvenile COTS and indirect effects through changes in their CCA food, COTS were grown in three pHT levels (7.9, 7.8, 7.6) and fed CCA grown at similar pH levels. Consumption of CCA by COTS was bolstered when the COTS were grown in low pH and when they were fed CCA grown in low pH regardless of the pH in which the COTS were reared. COTS fed CCA grown at pH 7.6 grew fastest, but the pH/pCO2 that the COTS were reared in had no direct effect on growth. Ocean acidification conditions decreased the C : N ratio and carbonate levels in the CCA. Bolstered growth in COTS may be driven by enhanced palatability, increased nutritive state and reduced defences of their CCA food. These results indicate that near future acidification will increase the success of early juvenile COTS and boost recruitment into the coral-eating life stage.

气候变化通过调控营养级相互作用所产生的间接效应，与气候胁迫对消费者的直接效应同等重要。长棘海星（crown-of-thorns starfish, COTS）*Acanthaster planci*的植食性幼体阶段的存活成功率，可能受到海洋环境对其结壳珊瑚藻（crustose coralline algae, CCA）食物的影响。为拆分近未来海洋酸化对幼体长棘海星的直接效应，以及通过其CCA食物变化产生的间接效应，实验设置了3个pHT梯度（7.9、7.8、7.6），将幼体长棘海星饲养于对应pH环境，并投喂饲养于相同pH梯度的结壳珊瑚藻。
无论幼体长棘海星自身的饲养pH条件如何，当其饲养于低pH环境，或投喂低pH环境下培育的结壳珊瑚藻时，其对CCA的取食速率均有所提升。投喂pH 7.6环境下培育的CCA的幼体长棘海星生长速率最快，但幼体长棘海星自身所处的pH/pCO₂分压并未对其生长产生直接影响。
海洋酸化条件会降低结壳珊瑚藻的碳氮比与碳酸盐含量。幼体长棘海星的生长速率提升，可能源于其CCA食物的适口性增强、营养状态改善以及防御能力下降。本研究结果表明，近未来的海洋酸化将提升幼体长棘海星早期的存活成功率，并推动其向食珊瑚生活阶段的种群补充。